Associations of Influenza Illness During Pregnancy with Pregnancy Loss and Program-Defined Child Health Monitoring Indicator: A Matched Cohort Study

Background/Objectives: This prospective cohort study assessed the impact of seasonal influenza during pregnancy on fetal and infant health outcomes. Methods: Pregnant women with laboratory-confirmed influenza were matched (1:4) by age and pregnancy loss history with women without influenza from annual cohorts in Suzhou, China, during 2015–2018. Participants underwent twice-weekly follow-up for influenza illness, and medical records were linked to ascertain outcomes. Multivariable regression models were used to estimate associations. Results: We included 441 pregnant women with influenza and 1764 without it. Four (0.9%) in the influenza group had late pregnancy loss, whereas one (0.1%) in the non-influenza group did. Influenza was associated with late pregnancy loss (adjusted hazard ratio [aHR] 31.1, 95% Confidence Interval [CI]: 1.3–756.8, p = 0.035). Four (0.9%) in the influenza group experienced early pregnancy loss, while five (0.3%) in the non-influenza group did. Influenza was not significantly associated with early pregnancy loss (aHR 2.1, 95% CI: 0.4–10.4, p = 0.374). By 8 months of age, infants born to 198 of the 441 mothers (44.9%) in the influenza group met the criteria for the program-defined child health monitoring indicator, compared with infants born to 650 of the 1764 mothers (36.8%) in the non-influenza group. Overweight/obesity was the largest contributor: 142 (32.2%) versus 470 (26.6%). Maternal influenza was associated with higher odds of meeting program-defined child health monitoring indicator in offspring (adjusted odds ratio [aOR] 1.4, 95% CI: 1.1–1.8, p = 0.006), with a significant contribution from overweight/obesity (aOR 1.3, 95% CI: 1.0–1.7, p = 0.045). Conclusions: In this cohort, influenza during pregnancy was associated with an increased risk of late pregnancy loss and program-defined child health monitoring indicator in offspring, particularly overweight/obesity. Because the observed association with pregnancy loss was based on a small number of events and had a wide confidence interval, it should be interpreted cautiously and confirmed in larger studies. These findings are consistent with current recommendations supporting influenza vaccination during pregnancy to protect maternal and infant health.